It has been the practice to mold sheet molding compound, commonly referred to in the art as SMC, in molds. SMC molding is widely used for high volume production of large, rigid plastic parts. For economic reasons, the bulk resin that fills the mold is usually of the less expensive resins and has inferior physical properties, such as poorer chemical resistance, as compared to the more costly resins. The bulk or fill resins in SMC molding especially have poor surface quality and are apt to exhibit blemishes, such that the molded products esthetically fail to meet consumer acceptance. For example, SMC moldings are apt to have sink marks, porosity, waviness, pits, dimples, and the like.
To overcome these faults, it has been suggested to paint the SMC molded products. This, however, only masks the more desirable resinous appearance. Also, painting does not effectively fill the more serious sink marks. It is, therefore, customary to inspect all parts and recycle defective ones through additional sanding, painting and further inspection until an acceptable appearance is obtained.
It has also been suggested to apply a coat about a previously molded SMC product which can be of the more expensive type of resin and thereby furnish the desired physical properties, at least to outward appearance and exposure. One process following this technique has been to compression mold an SMC product, open the mold, pour about the product a resinous coating compound, and then reclose the mold for a second heating or cure operation.
However, this technique has not been entirely satisfactory. After the initial compression molding and when the mold is opened to apply the resinous coating, it is necessary that the bulk resin portion does not unduly leave the mold or, if it does, that the molded bulk resin re-seats itself satisfactorily for the second heating step. If the re-seating is off, the coat covering is apt to be uneven and some areas of the SMC molding may not even receive some of the resin designed to form the coat. Further, the opening and closing of the mold tends to trap flash into the mold.
Still further, the described technique especially does not work well when the mold has sharp bends or intricate curving surface contours. In this case, tears occur in the after-applied coat as it attempts to adapt itself to irregular surface contours of the mold and draft of the mold press. In such cases, blemishes, and other defects may still appear.
In U.S. Pat. No. 3,917,790 to Oswitch et al, a dry, resinous mat which may be reinforced with fibers is cut to size and lined along a female mold. A bulk resin is then added to the mold which presses the mat into conformity with the mold shape. Thereafter, the resins are finally cured by heat.
U.S. Pat. No. 3,791,899 to Walters discloses placing a layer of plastic powder on a heated surface of a mold, heating the layer to make it tacky, and distributing compatible plastic pellets over the tacky layer. The pellets are heated until they soften. Upon cooling the mold, the pellets adhere to each other and to the layer.
U.S. Pat. No. 3,954,901 to Watanabe et al discloses a mixture of a polyester resin, a melamine and a blocked isocyanate resin sprayed and used as a powder paint.
U.S. Pat. No. 3,694,530 to Wolfe teaches the use of a solution of polyester resin to form a film for polyurethane foam.